The present invention relates to digital transmission by time division in which data are transmitted in the form of pulses grouped into time multiplexed words in the form of a reference frame and more particularly relates to the serial processing of these signals in terminal equipment.
In known multipath transmission methods the digital synchronous signals, for example, signals with a flow rate of 64 kilobits per second, are time-division multiplexed in an emitting station and are then transmitted on a multiplex digital path with a high flow rate, for example, at 2048 megabits per second, if multiplex digital path results from the multiplexing of 32 digital signals with 64 kilobits per second.
Multiplexed digital signals are conventionally produced from an analog signal by sampling, coding and logarithmic compression. The analog signal is made up from a finished group of frequencies, and usually comprises either one or two frequencies selected in said group. For example, in the case of signaling between switching offices, this signal consists of either two code frequencies or one control frequency.
Conventional signal detection methods involve restoring the analog signals and processing them with conventional filtering methods. Such a method requires the demultiplexing of the digital signal and its processing in digital-to-analog converters in order to restore an analog signal. This method calls for the path by path processing and for the same number of filters as there are frequencies within each path, each filter being adjusted separately. Such a method is referred to in the article by NIWA and SATO "Multifrequency receiver for pushbutton signaling using digital processing techniques," Proc. of National Electronics Conference, Chicago 1973.
Numerous articles have dealt with research on methods for the detection of multifrequency digital signals. One of these methods described in IEEE Transactions, Vol. 2, No. 12, December 1973, involves numerous operations, namely multiplications, squaring, and does not provide for the centralized processing of a reference frame.